Marking Seams with Mirror Modifier?
In the modifiers tab, any modifier that's in use has 4 buttons: Visible in Render, Visible in Object, Visible in Edit, and Vertex selectable in Edit mode. Press the last one, the one with the form of an inverted triangle. That should make the mirrored part be selectable in edit mode. Anyways, whatever you select there will also automatically be selected on the other part of the mesh that's the original mirrored and vice versa, because they are mirrored. Making seams on whichever of them will make the seam on the other, even if you did not select the mirrored vertex to be visible in edit mode.